1. Field of the Invention
This invention relates to an electroless nickel plating solution having improved fine patterning capability and a method for chemically depositing a nickel coating on a workpiece. It also relates to a high-build gold plating method capable of chemically depositing a thick gold coating on a chemically nickel-plated workpiece, which method is industrially advantageous in forming gold coatings on printed circuit boards and electronic parts.
2. Prior Art
Electroless or chemical nickel plating has been utilized in a wide variety of fields because of its advantageous features. For example, electroless nickel plating has been widely applied to electronic appliances. The electroless nickel plating technology, however, has not fully caught up with the urgent demand from the electronic appliance side.
The demand for reducing the weight of electronic appliances promoted to increase the density of constituent circuits, leading to finer circuit patterns. Several problems arise when conventional electroless plating solutions are used for plating on such fine patterns. A reduced line width gives rise to the problem that plating has a thin shoulder. A narrow pattern pitch gives rise to the problems of a reduced resistance between lines by plating projection or outgrowth and short-circuiting by a plating bridge. By the term "thin shoulder" it is meant that plating does not fully deposit on a shoulder of a circuit runner as viewed in cross section and the plating portion at the shoulder is significantly thinner than the remainder of plating. This is probably because the stabilizer excessively adheres to the shoulder to restrain metal deposition. By the term "plating outgrowth" it is meant that plating protrudes from metallic copper or circuit runners and a coating deposits around the circuit runners. This is probably because palladium ions left adhered around the circuit runners after palladium (activator) treatment are reduced with the electroless nickel plating solution into metallic palladium which exerts catalysis to help nickel deposit thereon.
Also, electroless gold plating is often used in the field of electronic industrial parts such as printed circuit boards, ceramic IC packages, ITO substrates, and IC cards since gold has many advantages including electric conduction, physical properties such as thermo-compression bonding ability, oxidation resistance, and chemical resistance. It is an important problem in the printed circuit board industry to chemically deposit thick gold coatings in an efficient manner.